1. Field of the Invention
The field of the present invention is optical tracking devices, particularly optical tracking devices that are used to determine the axial direction of an incoming beam of light.
2. Background
Optical tracking is a general purpose tool that can be used as an enabling technology in a broad range of applications. One example is determining the axial direction of a laser beam. For example, a suitably accurate optical tracker could be incorporated into a surface scanning device which, by sending out a laser beam towards a surface with unknown orientation and measuring the reflected beam's axial direction, determines the surface orientation with respect to the scanning device.
Optical tracking devices can be used aboard stationary or moving platforms to determine their position or orientation with respect to one or more light sources. Stellar tracking devices, commonly known in the art as a star sensor or a star tracker, are excellent examples of an application which permits the tracking of spacecraft orientation with respect to a known star field. Star trackers are critical components for space-based systems, regardless of whether such systems are purposed for terrestrial-based duties, e.g., detection, tracking, discrimination, or communication systems, or for non-terrestrial observations. The accuracy with which any of these duties may be performed from a space-based platform depends upon the accuracy of the position and orientation information available to the platform, and such information is most easily obtained from the tracking of stars (especially when used with a good GPS receiver).
A traditional star tracker images a star field using a controlled blur of star images to facilitate accurate pixel interpolation. Usually, each blurred star image resolves to an area of between 2×2 to 6×6 pixels on the image plane, and those pixels are processed to determine a local centroid for each star. Additional pixels around the resolved area are also frequently processed to provide data that may be used to correct for gradients in the background glow due to stray light. The typical centroid accuracy of traditional state of the art star trackers is 1/10th to 1/50th of a pixel. When a 2K×2K FPA is used to image a 20° field of view, the centroiding accuracy limits the directional accuracy to about 1-2 arcseconds. In order to improve on this accuracy, smaller fields of view are frequently used. However, a smaller field of view decreases the number of reliable star sources available for tracking and slows down the required measurement rate. The decreased field of view also reduces the slew rate capabilities of a star tracker, with the slew rate of currently available star trackers being no more than 1 deg/sec, and some being one or two orders of magnitude less. Moreover, the blurring technique used in such traditional star trackers makes the tracker highly dependent upon the spectral color of light emitted from stars. This results in star trackers that are less sensitive to certain stars, thereby limiting the number of stars useable for tracking purposes.
Traditional star tracking centroiding accuracy can be improved with high resolution FPA's. However, as the resolution of the FPA increases, so do the weight, volume, and power consumption of the star tracker. Moreover, with higher resolution FPAs, substantially more data must be processed, which leads to a greater weight for the star tracker and greater power being consumed by the processor tasked with crunching the data. Also, higher resolution FPAs tend to be more susceptible to radiation damage, with some of the most accurate star trackers currently available being damaged by as little as 15 krad to 20 krad of radiation exposure.